UVB radiation, vitamin D and multiple sclerosis

Achesonet al.(1960) observed an inverse relationship between sunlight exposure and the incidence of Multiple Sclerosis (MS).

Significance of proteolytic activity in 1,25-(OH)2D3-induced differentiation of HL-60 cells

Two types of HL-60 cells, 1,25-(OH)2D3-responsive ATCC HL-60 cells and 1,25-(OH)2D3-resistant LG HL-60 cells were used. Despite the presence of enough amounts of normal 1,25-(OH)2D3 receptors, only 22% of LG cells matured after a 4-day treatment with 10(-7) M 1,25-(OH)2D3, while 80% of ATCC cells differentiated. However, 1,25-(OH)2D3 inhibited the proliferation of LG cells to the same degree as that of ATCC cells. 1,25-(OH)2D3 also induced (1) the ability to metabolize 1,25-(OH)2D3 to 1,24,25-(OH)3D3, and (2) up-regulation of the 1,25-(OH)2D3 receptor in LG as well as ATCC cells. Furthermore, the proportion of mature LG cells was 78% after treatment with 10(-7) M 1,25-(OH)2D3 for the first 48 h and 10(-7) M dbcAMP for the second 48 h, which was greater than that when treated only with 10(-7) M dbcAMP for the second 48 h (24.2%). These results indicate that 1,25-(OH)2D3 receptor complexes function normally in LG cells at commitment step in cell differentiation. ATCC cells had a serine proteinase to destroy specific 1,25-(OH)2D3-binding activity of the unoccupied receptor and digest 53-kD receptor to a small fragment with a MW of 16.3 kD, while not affecting the level of the specific binding of the occupied receptor. Other cells, such as murine leukemia cells, M1, and human chronic myeloid leukemia cells, the differentiation of which is induced by 1,25-(OH)2D3, seemed to have the same type of proteinase, suggesting the physiological significance of this proteinase in 1,25-(OH)2D3-induced cell differentiation.

Regulation of the vitamin D endocrine system located in the kidney

During the past decade, it has been clearly established that the kidney is an endocrine organ which converts 25-hydroxyvitamin D to the active hormone 1,25-dihydroxyvitamin D. The kidney and intestine are the sites of an alternative hydroxylation which places an 24 hydroxyl in the R position on either 25-hydroxyvitamin D or 1,25-dihydroxyvitamin D. The production of 1,25-dihydroxyvitamin D is stimulated by hypocalcemia through the parathyroid gland, hypophosphatemia, the sex hormones and the peptide hormones insulin and prolactin. This vitamin D based endocrine system is of central importance in regulating calcium and phosphorus metabolism and its disturbances result in a variety of pathological conditions.

Triennial Growth Symposium--Vitamin D: bones and beyond

The exploration of vitamin D metabolism and function has led to the discovery of active forms of vitamin D that find great usefulness in treating patients with bone disease or renal failure and also perhaps in topical application for the treatment of skin disorders, such as psoriasis. It may also be effective in some types of autoimmune disease. This warrants our attention to maintaining an adequate vitamin D level in our blood to assure that the expected functions of vitamin D take place. However, we must not get so overenthusiastic as to expect vitamin D to be effective in treating or preventing many diverse diseases and especially caution is urged in considering that vitamin D compounds might be used to suppress cancerous growth.

2α-Methyl-19-nor-(20S)-1,25-dihydroxyvitamin D(3) protects the insulin 2 knockout non-obese diabetic mouse from developing type 1 diabetes without hypercalcaemia

Type 1 diabetes (T1D) is an autoimmune disease that destroys the insulin-producing beta-islet cells of the pancreas. Currently, there are no treatment modalities for prevention of T1D, and the mechanisms influencing disease inception and early progression are not well understood. We have used the insulin 2(-/-) non-obese diabetic (Ins2(-/-) NOD) model to study stages of T1D and to examine the protective effects of a potent analogue of 1α,25-dihydroxyvitamin D(3), 2α-methyl-19-nor-(20S)-1α,25-dihydroxyvitamin D(3) (2AMD). Pancreatic tissues from control and 2AMD-treated Ins2(-/-) NOD mice were obtained weekly from 5 to 16 weeks of age. Using immunohistochemical (IHC) analysis, samples were analysed for changes in beta cell survival, islet structure and T cell invasion. Weekly intraperitoneal glucose tolerance tests (IPGTT) were performed to assess comparative beta cell function in control and treated animals. IHC demonstrated progressive beta cell destruction in control mice. In contrast, 2AMD treatment preserved islet cell architecture, arrested intra-islet T cell invasion and prevented the transition from insulitis to diabetes. IPGTT results revealed progressive impairment of beta cell function with increasing age in control mice, while 2AMD treatment resulted in normal beta function throughout the study. These results demonstrate that the Ins2(-/-) NOD model provides a rapid and effective method for studying T1D and for assessing efficacy of anti-diabetic agents.

2MD, a new anabolic agent for osteoporosis treatment

INTRODUCTION

2-Methylene-19-nor-(20S)-1alpha,25-dihydroxyvitamin D3 (2MD) is a new analog of 1alpha,25-dihydroxyvitamin D3 (1,25-(OH)2D3) that has unique properties (distinct from 1alpha,25-dihydroxyvitamin D3) in stimulating osteoblasts to form bone in culture. This analog has now been extensively tested in aged ovariectomized female rats maintained on a diet adequate in calcium and phosphorus.

METHODS

Retired female rats obtained from Sprague-Dawley were ovariectomized, and were either dosed with vehicle or 2MD at 5-7 ng/kg body weight each day.

RESULTS

A marked increase in total bone mass resulted during the 28-week study. This increase in bone mass resulted from an increase in both cortical and trabecular bone, with increases to the order of 25% in the cancellous bone. Histomorphometry revealed that 2MD increased bone mass primarily by increasing bone formation. It also revealed little or no effect on bone resorption. The resulting bone is of high quality revealed by histology and biomechanical testing.

CONCLUSION

Throughout the study, serum calcium remained within the normal range and thus 2MD shows great promise for the treatment of bone diseases characterized by bone loss, including osteoporosis.

Assay variation confounds the diagnosis of hypovitaminosis D: a call for standardization

Endemic hypovitaminosis D contributes to osteoporosis development. However, variation in 25-hydroxyvitamin D (25OHD) measurement is reported and confounds the diagnosis of vitamin D insufficiency/deficiency. This report emphasizes the marked variability observed in serum 25OHD measurements between laboratories.Initially, postmenopausal women had serum 25OHD determinations: 42 in laboratory A, 20 in laboratory B. Their mean (sem) serum 25OHD concentrations were 46 (2.1) and 21 (2.3) ng/ml in laboratories A and B, respectively. Furthermore, there was little overlap in serum 25OHD among these clinically similar individuals. Specifically, 17% of those measured in laboratory A but 90% in laboratory B were below an arbitrary threshold value of 32 ng/ml.Subsequently, serum was obtained from 10 healthy adults. Two aliquots from each individual, one of which was spiked with 20 ng/ml 25OHD, were sent to six laboratories. Substantial variability was noted between these six laboratories. The mean serum 25OHD concentration ranged from 17.1-35.6 ng/ml. Similarly, the mean increase produced by spiking with 20 ng/ml ranged from 7.7-18.0 ng/ml.In conclusion, 25OHD assays yield markedly differing results; whether an individual is found to have low or normal vitamin D status is a function of the laboratory used. If the medical community is to make progress in correcting widespread hypovitaminosis D, 25OHD measurement must be standardized.

Novel synthesis of 2-substituted 19-norvitamin D A-ring phosphine oxide from D-glucose as a building block

19-norvitamin D A-ring phosphine oxide 5 was synthesized by a new sequence mode starting from D-glucose as a chiral template. Transformation of the pyranoside ring into the A-ring carbocycle was achieved by the Pd-catalyzed Ferrier rearrangement. The phosphine oxide 5 was obtained in an 18% overall yield by this novel cost-effective method.

Moderate cholecalciferol supplementation depresses intestinal calcium absorption in growing dogs

Hormonal regulation of calcium (Ca) absorption was investigated in a cholecalciferol (vitamin D(3))-supplemented group (hVitD) vs. a control group (cVitD) of growing Great Danes (100 vs. 12.5 micro g vitamin D(3)/kg diet). Although Ca intakes did not differ, fractional Ca absorption was significantly lower in the hVitD group than in the cVitD group. There were no differences in plasma concentrations of Ca, inorganic phosphate, parathyroid hormone, growth hormone or insulin-like growth factor I between groups. Plasma 25-hydroxycholecalciferol [25(OH)D(3)] concentrations were maintained in the hVitD dogs at the same levels as in the cVitD dogs due to increased turnover of 25(OH)D(3) into 24,25-dihydroxycholecalciferol [24,25(OH)(2)D(3)] and 1,25-dihydroxycholecalciferol [1,25(OH)(2)D(3)]. In hVitD dogs, the greater plasma 24,25(OH)(2)D(3) concentration and the enhanced metabolic clearance rate (MCR) of 1,25(OH)(2)D(3) indicated upregulated 24-hydroxylase activity. The increased MCR of 1,25(OH)(2)D(3) decreased plasma 1,25(OH)(2)D(3) concentrations. In hVitD dogs, the greater production rate of 1,25(OH)(2)D(3) was consistent with the 12.9-fold greater renal 1alpha-hydroxylase gene expression compared with cVitD dogs and compensated to a certain extent for the accelerated MCR of 1,25(OH)(2)D(3). The moderately decreased plasma 1,25(OH)(2)D(3) concentration can only partially explain the decreased Ca absorption in the hVitD dogs. Intestinal vitamin D receptor concentrations did not differ between groups and did not account for the decreased Ca absorption. We suggest that 24,25(OH)(2)D(3) may downregulate Ca absorption.

Parathyroid hormone regulates 25-hydroxyvitamin D(3)-24-hydroxylase mRNA by altering its stability

The up-regulation of the 25-hydroxyvitamin D(3)-24-hydroxylase by 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] is well established and occurs at the transcriptional level through two vitamin D response elements in the promoter of the gene. However, the mechanism of down-regulation of the 24-hydroxylase by parathyroid hormone (PTH) has not yet been elucidated. To study the mechanism of PTH action, we used AOK-B50 cells, a porcine kidney-cell line with stably transfected opossum PTH receptor in which both the 24-hydroxylase mRNA and activity are down-regulated by PTH. Cells dosed with 1,25(OH)(2)D(3) at 0 h, and subsequently at 0, 1, 2, or 4 h with 100 nM of PTH, showed levels of 24-hydroxylase mRNA equivalent to 72.6, 65.3, 57.2, and 37.1%, respectively, of the levels found in cells dosed with 1,25(OH)(2)D(3) only. All cells were collected 7 h after the initial 1,25(OH)(2)D(3) dose. This pattern of expression indicated that PTH does not act by repressing transcription but rather by making the mRNA for 24-hydroxylase susceptible to degradation. At least 1 h is required for PTH to act. Further RNA and protein syntheses are required for PTH to act. However, the sites and mechanism whereby PTH causes 24-hydroxylase mRNA degradation are unknown. Because the untranslated regions of genes can determine the stability of its transcripts, we studied the 5' untranslated region and the 3' untranslated region of the rat 24-hydroxylase gene by using reporter-gene strategy to identify possible PTH sites of action. None was found, suggesting that the destabilization site is elsewhere in the coding region.

Metabolism of a physiological amount of all-trans-retinol in the vitamin A-deficient rat

Because only retinol and not all-trans-retinoic acid (atRA) can satisfy all of the functions of vitamin A, we have investigated the retinol metabolites in tissues of vitamin A-deficient (VAD) rats responding to a radioactive dose of [20-(3)H]all-trans-retinol. As expected, atRA is the major vitamin A metabolite present in the target tissues of VAD rats given a physiological dose (1 microg) of [20-(3)H]all-trans-retinol (atROL). Both atROL and atRA were detected by high-performance liquid chromatographic (HPLC) analysis of the radioactivity extracted from the liver, kidney, small intestine, lung, spleen, bone, skin, or testis of these animals. Novel retinol metabolites were observed in the aqueous extracts from the testis, lung, and skin. However, these metabolites were detected in very small amounts and were not characterized further. Importantly, neither 9-cis-retinoic acid (9cRA), 9-cis-retinol (9cROL), nor 13-cis-retinoic acid (13cRA) was present in detectable amounts. The amounts of atRA varied in each tissue, ranging from 0.29 +/- 0.05 fmol of RA/g of tissue in the femurs to 12.9 +/- 4.3 fmol of RA/g of tissue in the kidneys. The absence of 9cRA in vivo was not due to degradation of this retinoid during the extraction procedure or HPLC analysis of the extracted radioactivity. As atROL completely fulfills all of the physiological roles of vitamin A, and 9cRA is not detected in any of the tissues analyzed, these results suggest that 9cRA may have no physiological relevance in the rat.

Photoactivable analogs for labeling 25-hydroxyvitamin D3 serum binding protein and for 1,25-dihydroxyvitamin D3 intestinal receptor protein

3-Azidobenzoates and 3-azidonitrobenzoates of 25-hydroxyvitamin D3 as well as 3-deoxy-3-azido-25-hydroxyvitamin D3 and 3-deoxy-3-azido-1,25-dihydroxyvitamin D3 were prepared as photoaffinity labels for vitamin D serum binding protein and 1,25-dihydroxyvitamin D3 intestinal receptor protein. The compounds prepared were easily activated by short- or long-wavelength uv light, as monitored by uv and ir spectrometry. The efficacy of the compounds to compete with 25-hydroxyvitamin D3 or 1,25-dihydroxyvitamin D3 for the binding site of serum binding protein and receptor, respectively, was studied to evaluate the vitamin D label with the highest affinity for the protein. The presence of an azidobenzoate or azidonitrobenzoate substituent at the C-3 position of 25-OH-D3 significantly decreased (10(4)- to 10(6)-fold) the binding activity. However, the labels containing the azido substituent attached directly to the vitamin D skeleton at the C-3 position showed a high affinity, only 20- to 150-fold lower than that of the parent compounds with their respective proteins. Therefore, 3-deoxy-3-azidovitamins present potential ligands for photolabeling of vitamin D proteins and for studying the structures of the protein active sites.

Model of three-dimensional structure of vitamin D receptor and its binding mechanism with 1alpha,25-dihydroxyvitamin D(3)

Comparative modeling of the vitamin D receptor three-dimensional structure and computational docking of 1alpha,25-dihydroxyvitamin D(3) into the putative binding pocket of the two deletion mutant receptors: (207-423) and (120-422, Delta [164-207]) are reported and evaluated in the context of extensive mutagenic analysis and crystal structure of holo hVDR deletion protein published recently. The obtained molecular model agrees well with the experimentally determined structure. Six different conformers of 1alpha,25-dihydroxyvitamin D(3) were used to study flexible docking to the receptor. On the basis of values of conformational energy of various complexes and their consistency with functional activity, it appears that 1alpha,25-dihydroxyvitamin D(3) binds the receptor in its 6-s-trans form. The two lowest energy complexes obtained from docking the hormone into the deletion protein (207-423) differ in conformation of ring A and orientation of the ligand molecule in the VDR pocket. 1alpha,25-Dihydroxyvitamin D(3) possessing the A-ring conformation with axially oriented 1alpha-hydroxy group binds receptor with its 25-hydroxy substituent oriented toward the center of the receptor cavity, whereas ligand possessing equatorial conformation of 1alpha-hydroxy enters the pocket with A ring directed inward. The latter conformation and orientation of the ligand is consistent with the crystal structure of hVDR deletion mutant (118-425, Delta [165-215]). The lattice model of rVDR (120-422, Delta [164-207]) shows excellent agreement with the crystal structure of the hVDR mutant. The complex obtained from docking the hormone into the receptor has lower energy than complexes for which homology modeling was used. Thus, a simple model of vitamin D receptor with the first two helices deleted can be potentially useful for designing a general structure of ligand, whereas the advanced lattice model is suitable for examining binding sites in the pocket.

Analysis of basal regulatory elements in the 25-hydroxyvitamin D3 1alpha-hydroxylase gene promoter

5'-Deletion analysis of the 1.7-kb mouse 1alpha-hydroxylase gene promoter reveals that the minimal promoter region for basal activity is -85/+22 and requires a functional CCAAT element. Mutational analysis also demonstrates that deletion of the internal promoter region from nucleotides -1125 to -86 leads to a 25- to 30-fold increase in basal promoter activity. The increased activity is not the result of positional effects, but is caused in part by the removal of an AC repeat. Further analysis of the promoter revealed an enhancer element localizes to an upstream region -1385 to -1125, which contains three consensus AP-1 sites. Deletion of the most proximal AP-1 site causes a 60% loss of enhancer activity. The data suggest the presence of the AC repeat prevents the full potential activation of the 1alpha-hydroxylase promoter by factors binding to AP-1 sites.

Vitamin D receptor null mutant mice fed high levels of calcium are fertile

Vitamin D receptor (VDR) null mutant mice provide a model to investigate the possible effect of vitamin D on female reproduction. Infertility in these mice has been reported but it is uncertain whether the infertility results from a lack of VDR or from the hypocalcemia that results from a lack of VDR. VDR null mutant mice and wild-type controls were fed a nonpurified, high calcium or medium calcium diet, plus a diet containing lactose and their reproductive efficiency was examined. VDR null mutant mice fed a nonpurified diet were hypocalcemic and were found to be largely infertile with 14% fertility, while the fertility percentage of normocalcemic VDR null mutant mice and wild-type mice was between 86% and 100%. A high calcium or medium calcium diet maintained 100% fertility in the VDR knockout mice; removal of the lactose from this diet did not diminish reproductive capability. Reproductive capacity of VDR null mutant mice was analyzed when they were fed purified diets containing 0.02-2% calcium. Mutant mice fed a low calcium diet (0.47%) had a lower reproductive efficiency than VDR null mutant mice fed a diet that resulted in normal serum calcium concentrations. Thus, high dietary calcium levels are required for normal reproduction in VDR null mutant female mice. It seems that the defect in reproduction reported previously for VDR null mutant mice is not the lack of a direct effect of 1,25-dihydroxycholecalciferol on reproductive function but is the result of hypocalcemia.

26-Hydroxylation of 1α,25-Dihydroxyvitamin D3 Does Not Occur under Physiological Conditions

The 26-hydroxylation of 1alpha,25-dihydroxyvitamin D3 in rats in vitro and in vivo was studied under physiological conditions. Incubation of 1alpha,25-dihydroxy-[26,27-3H]vitamin D3 with rat kidney or rat liver homogenate showed formation of a metabolite that was identified as 1alpha,25(S),26-trihydroxy-[26,27-3H]vitamin D3 by comigration on three different HPLC systems and a periodate cleavage reaction. This metabolite was not generated by hydroxylation of 1alpha,25-dihydroxy-[26,27-3H]vitamin D3 itself but by an enzymatic conversion of a precursor that was formed nonenzymatically in substantial amounts upon storage of 1alpha,25-dihydroxy-[26,27-3H]vitamin D3 in ethanol at -20 degrees C under argon for more than three weeks. An in vivo metabolism study in rats dosed with a physiological dose of 1alpha,25-dihydroxy-[26,27-3H]vitamin D3 confirmed the absence of 26-hydroxylation of the hormone. As expected at 6 h postinjection of purified 1alpha,25-dihydroxy-[26,27-3H]vitamin D3, 1alpha,24(R),25-trihydroxy-[26,27-3H]vitamin D3, as well as traces of (23S,25R)-1alpha,25-dihydroxy-[3H]vitamin D3-lactone were detected and identified on straight phase and reverse phase HPLC in serum, kidney, and liver.

Systematic studies on synthesis, structural elucidation, and biological evaluation of A-ring diastereomers of 2-methyl-1alpha,25-dihydroxyvitamin D(3) and 20-epi-2-methyl-1alpha,25-dihydroxyvitamin D(3)

All possible A-ring diastereomers of 2-methyl-1alpha,25-dihydroxyvitamin D(3) (2) and 20-epi-2-methyl-1alpha,25-dihydroxyvitamin D(3) (3) were synthesized by palladium-catalyzed coupling reaction of A-ring 'enyne' synthons with CD-ring portions. The A-ring synthons were rationally synthesized via a novel and practical route, starting with methyl (R)-(+)- and (S)-(-)-3-hydroxy-2-methyl-propionate, in good yields. X-ray crystallographic analysis of 2alpha-methyl-1alpha,25-dihydroxyvitamin D(3) (2b) and conformational analysis of the A-ring of 2alpha-methyl-(2b) and 2beta-methyl-1alpha,25-dihydroxyvitamin D(3) (2f) were carried out, and the results are described. All A-ring diastereomers (2 and 3), thus synthesized, were biologically evaluated both in vitro and in vivo. The biologic potency was highly dependent on the stereochemistry of the A-ring substituents. In particular, 2b showed 4-fold higher vitamin D receptor [VDR] binding activity than the natural hormone, and its 20-epimer (3b) exhibited exceptionally high activity, 12-fold more potent in VDR binding, 7-fold in calcium mobilization, and 590-fold in induction of human promyelocytic leukemia (HL-60) cell differentiation as compared with the natural hormone. Further, the 20-epi-2beta-Me-1beta, 3alpha(OH)(2) isomer (3g) had significant biologic potencies compared to the natural hormone despite having 1beta-OH configuration. The transcriptional activities on human osteocalcin gene promoter, including VDRE in transfected mammalian cells, were also evaluated. Finally, there was a clear contrast between the effects of the 2-methyl group on the HL-60 cell differentiation- and apoptosis-inducing activities of 2 and 3.

The Wilms' tumor gene product (WT1) modulates the response to 1,25-dihydroxyvitamin D3 by induction of the vitamin D receptor

The Wilms' tumor gene (wt1) encodes a transcription factor involved in urogenital development, in particular in renal differentiation, and in hematopoietic differentiation. Differentiation of a number of solid tumor and leukemic cells lines can be mediated by 1,25-dihydroxyvitamin D(3). This is predominantly mediated by the nuclear receptor for 1,25-dihydroxyvitamin D(3), the vitamin D receptor (VDR). In initial experiments addressing a possible link between WT1 and VDR, we observed a correlated expression of WT1 and VDR mRNA in samples from renal tissues. HT29 colon carcinoma cells, stably transfected to express WT1, exhibited elevated endogenous VDR levels compared with control cells transfected with a control construct. Elevated VDR expression was found in wt1-transfected human embryonic kidney 293 cells, as well. In transient cotransfection experiments, we observed an activation of a vdr promoter reporter by WT1 through a WT1 recognition element, indicating transcriptional regulation of the vdr gene expression by WT1. The responsive sequence element was specifically bound by wild-type, but not by mutated WT1, in electrophoretic mobility shift assays. HT29 colon carcinoma cells, which respond to 1,25-dihydroxyvitamin D(3) with slow induction of growth arrest, were investigated for the influence of WT1 on 1,25-dihydroxyvitamin D(3)-mediated growth suppression. Although HT29 cells transfected with a control construct responded moderately to 1,25-dihydroxyvitamin D(3), the response of HT29 cells expressing WT1 was strikingly enhanced. Stimulation with dihydroxyvitamin D(3) caused an up to 3-fold reduction in the growth rate of different HT29 clones expressing WT1 as compared with control cells lacking WT1 expression. Thus, induction of VDR by WT1 leads to an enhanced response to 1,25-dihydroxyvitamin D(3). We conclude that the vitamin D receptor gene is a target for transcriptional activation by WT1, suggesting a possible physiological role of this regulatory pathway.

Synthesis, biological evaluation, and conformational analysis of A-ring diastereomers of 2-methyl-1,25-dihydroxyvitamin D(3) and their 20-epimers: unique activity profiles depending on the stereochemistry of the A-ring and at C-20

All eight possible A-ring diastereomers of 2-methyl-1, 25-dihydroxyvitamin D(3) (2) and 2-methyl-20-epi-1, 25-dihydroxyvitamin D(3) (3) were convergently synthesized. The A-ring enyne synthons 19 were synthesized starting with methyl (S)-(+)- or (R)-(-)-3-hydroxy-2-methylpropionate (8). This was converted to the alcohol 14 as a 1:1 epimeric mixture in several steps. After having been separated by column chromatography, each isomer led to the requisite A-ring enyne synthons 19 again as 1:1 mixtures at C-1. Coupling of the resulting A-ring enynes 20a-h with the CD-ring portions 5a,b in the presence of a Pd catalyst afforded the 2-methyl analogues 2a-h and 3a-h in good yield. In this way, all possible A-ring diastereomers were synthesized. The synthesized analogues were biologically evaluated both in vitro and in vivo. The potency was highly dependent on the stereochemistry of each isomer. In particular, the alpha alpha beta-isomer 2g exhibited 4-fold higher potency than 1 alpha,25-dihydroxyvitamin D(3) (1) both in bovine thymus VDR binding and in elevation of rat serum calcium concentration and was twice as potent as the parent compound in HL-60 cell differentiation. Furthermore, its 20-epimer, that is, 20-epi-alpha alpha beta 3g, exhibited exceptionally high activities: 12-fold higher in VDR binding affinity, 7-fold higher in calcium mobilization, and 590-fold higher in HL-60 cell differentiation, as compared to 1 alpha,25-dihydroxyvitamin D(3) (1). Accordingly, the double modification of 2-methyl substitution and 20-epimerization resulted in unique activity profiles. Conformational analysis of the A-ring by (1)H NMR and an X-ray crystallographic analysis of the alpha alpha beta-isomer 2g are also described.

Regulation of the procine 1,25-dihydroxyvitamin D3-24-hydroxylase (CYP24) by 1,25-dihydroxyvitamin D3 and parathyroid hormone in AOK-B50 cells

The 24-hydroxylase is the enzyme responsible for the first step in the catabolism of 1,25-dihydroxyvitamin D3, the active form of vitamin D. This enzyme was shown to be upregulated by 1,25-dihydroxyvitamin D3 itself and downregulated by parathyroid hormone (PTH). Upregulation of 24-hydroxylase by 1,25-dihydroxyvitamin D3 has been characterized; however, the mechanism by which PTH acts to downregulate 24-hydroxylase expression remains unknown. Here we report the cloning of the porcine 24-hydroxylase, and show that 1,25-dihydroxyvitamin D3-stimulated 24-hydroxylase mRNA and activity are repressed by PTH in AOK-B50 cells, a porcine kidney proximal tubule cell line with stably transfected opossum PTH receptors. Forskolin mimicked the effects of PTH consistent with in vivo data, and suppression by PTH was not due to changes in VDR levels. The first 1400 bp of the 24-hydroxylase promoter were not able to mediate the effects of PTH on a reporter gene. In view of the above findings we concluded that AOK-B50 cells are a suitable model for further studying the mechanism of action of PTH on 24-hydroxylase mRNA.

Regulation of 25-hydroxyvitamin D3 1alpha-hydroxylase gene expression by parathyroid hormone and 1,25-dihydroxyvitamin D3

The conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) takes place mainly in the kidney and is catalyzed by the enzyme 1alpha-hydroxylase. Parathyroid hormone (PTH) and 1,25-(OH)2D3 are well-known regulators of this tightly controlled step, but the mechanisms by which they modulate 1alpha-hydroxylase activity have not been fully delineated. Northern analysis showed PTH and forskolin rapidly and transiently increase 1alpha-hydroxylase expression in AOK-B50 cells and HKC-8 cells. Actinomycin D treatment blocks the increase, but cycloheximide does not. No decrease of 1alpha-hydroxylase transcript by 1,25-(OH)2D3 was observed in either cell line, although 24-hydroxylase levels were strongly induced by 1,25-(OH)2D3 treatment. 1,25-(OH)2D3 suppressed the 1alpha-hydroxylase transcript in vivo both in the presence and absence of exogenously supplied PTH. These results suggest that the stimulatory action of PTH is directly on the 1alpha-hydroxylase gene, while the repressive action of 1,25-(OH)2D3 does not involve the parathyroid gland but is nevertheless indirect.

Cloning of the mouse 25-hydroxyvitamin D(3)-1alpha-hydroxylase (CYP1alpha) gene

A genomic clone for 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-hydroxylase) was isolated from a mouse embryonic stem cell P1 genomic library. It contains nine exons spanning 4.8 kb from the transcriptional start site. All the intron insertion sites are identical to that of the human 1alpha-hydroxylase and human vitamin D(3) 25-hydroxylase genes. A polyadenylation signal AUUAAA that differs from the consensus sequence at the second residue was identified 16 bp downstream of the 3' end of the previously reported mouse cDNA. This element is the only common natural variant described. The 3' end of the gene was determined using a RACE technique. Three poly(A) addition sites were observed 12, 15 and 22 bases from the AUUAAA element. Such distances are in agreement with what is required for maturation of mammalian pre-mRNAs. This molecular cloning makes possible the generation of transgenic mice in order to further investigate the role and importance of the 25-hydroxyvitamin D(3)-1alpha-hydroxylase (CYP1alpha).

The mouse vitamin D receptor is mainly expressed through an Sp1-driven promoter in vivo

The availability of the mouse vitamin D receptor (mVDR) gene has allowed a characterization of a TATA-less promoter containing a cluster of four Sp1 sites named Sp1-1, Sp1-2, Sp1-3, and Sp1-4 (F. Jehan and H. F. DeLuca, 1997, Proc. Natl. Acad. Sci. USA 94, 10138-10143). By means of primer extension analysis, S1 nuclease mapping and ribonuclease protection assay, the start site has been deduced, as has the existence of other minor transcription start sites. Initiation of transcription at the major site is located 4 bp upstream of the 5' end of the mVDR cDNA sequence and very close to the putative Sp1 sites. A second minor promoter might exist between exon 1 and exon 2 of the mVDR gene. The nucleotide sequence of the Sp1 region is well conserved between the mouse, the human, and the chicken VDR genes, suggesting an important role for these Sp1 sites. Gel shift analysis of the four Sp1 sites of the mVDR promoter has confirmed specific binding complexes to Sp1-1, Sp1-2, and Sp1-4 (Sp1-3 rather binds an unknown complex that is unable to bind the canonical Sp1 GGGGCGGGGC). Deletion or mutation of all the Sp1 sites eliminates promoter activity. However, mutation or deletion of individual Sp1 sites did not dramatically change the promoter activity, except for mutation of Sp1-3 that increases promoter activity. We, therefore, conclude that the mVDR promoter is controlled by the Sp1 sites and is the main VDR promoter in intestine and kidney.

In vivo upregulation of interleukin-4 is one mechanism underlying the immunoregulatory effects of 1,25-dihydroxyvitamin D(3)

The active form of vitamin D (1,25-(OH)(2)D(3)) is a potent immune system regulator. In vivo the oral administration of 1, 25-(OH)(2)D(3) completely prevents experimental autoimmune encephalomyelitis (EAE), significantly prolongs allograft survival, and prevents collagen-induced arthritis. 1,25-(OH)(2)D(3) given to mice increased IL-4 protein and transcript levels. We have now tested the efficacy of 1,25-(OH)(2)D(3) on EAE development and allograft survival in IL-4-deficient [knockout (ko)] mice. 1, 25-(OH)(2)D(3) was found to be much less effective in the absence of IL-4, suggesting that IL-4 production is a significant factor in the action of 1,25-(OH)(2)D(3) on the immune system.

Metabolism of 1alpha,25-dihydroxyvitamin D(3) in vitamin D receptor-ablated mice in vivo

The metabolism of 1alpha,25-dihydroxyvitamin D(3) was studied in vitamin D receptor-ablated mice following the administration of a physiological dose of 1alpha,25-dihydroxy-[26,27-(3)H]vitamin D(3). The degradation of 1alpha,25-dihydroxy-[26,27-(3)H]vitamin D(3) in the vitamin D receptor null mutant mice was substantially reduced compared to the wild-type control mice. At 24 h postadministration of radiolabeled 1alpha,25-dihydroxyvitamin D(3) more than 50% of the radioactivity was recovered unmetabolized, whereas in wild-type mice nearly all of the 1alpha,25-dihydroxy-[26,27-(3)H]vitamin D(3) was degraded. In wild-type mice three polar metabolites other than 1alpha,25-dihydroxyvitamin D(3) were detected and identified on straight-phase and reverse-phase high-performance liquid chromatography as 1alpha,24(R),25-trihydroxy-[26,27-(3)H]vitamin D(3), 1alpha,25(S),26-trihydroxy-[26,27-(3)H]vitamin D(3), and (23S, 25R)-1alpha,25-dihydroxy-[(3)H]vitamin D(3)-26,23-lactone. Only one metabolite was detected in the plasma and kidneys of vitamin D receptor null mutant mice at 3 h following an intrajugular dose of 1alpha,25-dihydroxy-[26,27-(3)H]vitamin D(3). This metabolite was clearly identified as 1alpha,25(S),26-trihydroxy-[26,27-(3)H]vitamin D(3) by comigration on two HPLC systems and periodate cleavage reaction. At 6, 12, and 24 h postinjection 1alpha,24(R), 25-trihydroxy-[26,27-(3)H]vitamin D(3) was also detected at low levels in plasma, kidneys, and liver of some but not all mutant mice. The presence of 25-hydroxyvitamin D(3)-24-hydroxylase mRNA in the kidneys of these vitamin D receptor null mutant mice was confirmed by ribonuclease protection assay.

Three-dimensional modeling of and ligand docking to vitamin D receptor ligand binding domain

The ligand binding domain of the human vitamin D receptor (VDR) was modeled based on the crystal structure of the retinoic acid receptor. The ligand binding pocket of our VDR model is spacious at the helix 11 site and confined at the beta-turn site. The ligand 1alpha, 25-dihydroxyvitamin D(3) was assumed to be anchored in the ligand binding pocket with its side chain heading to helix 11 (site 2) and the A-ring toward the beta-turn (site 1). Three residues forming hydrogen bonds with the functionally important 1alpha- and 25-hydroxyl groups of 1alpha,25-dihydroxyvitamin D(3) were identified and confirmed by mutational analysis: the 1alpha-hydroxyl group is forming pincer-type hydrogen bonds with S237 and R274 and the 25-hydroxyl group is interacting with H397. Docking potential for various ligands to the VDR model was examined, and the results are in good agreement with our previous three-dimensional structure-function theory.

Expression of 1,25-dihydroxyvitamin D(3) receptor in the immune system

In addition to its role in calcium and skeletal homeostasis, there is increasing evidence that the hormonal form of vitamin D, 1, 25-dihydroxyvitamin D(3), appears to serve as a modulator of the immune system. We have determined the level of the 1, 25-dihydroxyvitamin D(3) receptor (VDR) in resting and activated lymphocytes by immuno- and ligand-binding assays. As expected from previous work, the total T lymphocyte population contains VDR whose levels are increased when activated and treated with 1, 25-dihydroxyvitamin D(3). Surprisingly, the highest concentrations of VDR are found in CD8 lymphocytes, although significant amounts are also present in CD4 lymphocytes. Furthermore, B lymphocytes do not contain detectable amounts of VDR. Cells of the monocyte/macrophage lineage possess small amounts of VDR that are not affected by activation but are increased by treatment with 1, 25-dihydroxyvitamin D(3). These results suggest that CD8 lymphocytes may be a major site of 1,25-dihydroxyvitamin D(3) action, while B lymphocytes are likely not directly regulated by 1, 25-dihydroxyvitamin D(3).

Isolation and characterization of the chicken vitamin D receptor gene and its promoter

The sequences from several independent cDNA clones encoding the chicken vitamin D receptor as well as primer extension assay have clearly delineated the 5' terminus and the transcriptional start site. Screening a chicken genomic library produced genomic clones containing vitamin D receptor (VDR) gene fragments. Restriction map of clone 8 showed that the 18.6-kb chicken VDR fragment has exons 1 and 2, intron 1, part of intron 2, and 7-kb 5' flanking region. Exons 1, 2, and 3 found in the chicken VDR gene shares low homology with its mammalian counterparts (i.e., E1A, E1B, and E1C in human). By contrast, the fourth exon and following exons for the coding region of VDR gene are highly conserved between avian and mammalian species. While the fourth exon bears the ATG sites for translation initiation in mammals, the third exon in birds has two extra ATG sites for leaky translation as determined previously. Thus, the avian VDR has more N-terminal sequence than the mammalian VDR and is found in two distinct forms. The 5' flanking region from genomic clone 8 shares considerable homology in several regions with the human and mouse VDR promoters. Moreover, the 5' flanking region of chicken VDR gene possesses promoter activity, as shown by its ability to drive the luciferase reporter gene in cell transfection assays. Like other steroid receptor promoters, the chicken VDR promoter contains no TATA box but possesses several GC boxes or SP1 sites. A series of deletional promoter constructs established that the proximal GC boxes are the major drivers of gene transcription, while the more upstream sequences have repressive elements.

Mechanism of action of superactive vitamin D analogs through regulated receptor degradation

We and others have previously shown that selected vitamin D analogs potentiate the vitamin D receptor (VDR) mediated transcription much more efficiently than the natural hormone itself. Here we show that the transcriptionally active 20-epi analogs, namely KH 1060 and MC 1288, protect VDR against degradation more efficiently than calcitriol at 10(-10) M concentration (VDR t(1/2) > 48 h, 17 h, and 10 h, respectively). The conformationally epi-like analog EB 1089 did not significantly alter the half-life of VDR (10.3 h), but retained the VDR levels over longer periods of time than calcitriol. The transcriptionally weak analog GS 1558, on the other hand, enhanced VDR degradation even more than what was observed with the unliganded receptor (t(1/2) 4.5 h and 5 h, respectively). Inhibition of proteasome activity by the inhibitor MG-132 resulted in a marked increase in the VDR levels in cells treated with the vehicle or GS 1558 (2.5-fold and 2.7-fold, respectively), more than twice the levels observed in the presence of calcitriol or EB 1089 (1.2-fold and 1.1-fold, respectively). MG-132 treatment did not increase the VDR levels in cells treated with KH 1060 or MC 1288. The electrophoretic mobility shift assay (EMSA) with nuclear extracts from MG-132-treated cells revealed formation of a high-molecular-weight RXRbeta-VDR-VDRE complex, which also contained Sug1. In the presence of calcitriol, 34% of total VDR in its DNA binding state was present in this complex. The 20-epi analogs effectively prevented the formation of this complex, since, in this case, only 16% of total VDR was found in this complex. These results suggest that KH 1060 and MC 1288 induce a VDR conformation, which prevents binding of proteins mediating receptor degradation. As a result, the regulation of VDR degradation differs from that found with the calcitriol-VDR complex resulting in superactive transcriptional action of the analogs.

Synthesis and biological activity of 22-iodo- and (E)-20(22)-dehydro analogues of 1alpha,25-dihydroxyvitamin D3

Construction of 25-hydroxy-steroidal side chain substituted with iodine at C-22 was elaborated on a model PTAD-protected steroidal 5,7-diene and applied to a synthesis of (22R)- and (22S)-22-iodo-1alpha,25-dihydroxyvitamin D3. Configuration at C-22 in the iodinated vitamins, obtained by nucleophilic substitution of the corresponding 22S-tosylates with sodium iodide, was determined by comparison of their iodine-displacement processes and cyclizations leading to isomeric five-membered (22,25)-epoxy-1alpha-hydroxyvitamin D3 compounds. Also, 20(22)-dehydrosteroids have been obtained and their structures established by 1H NMR spectroscopy. When compared to the natural hormone, (E)-20(22)-dehydro-1alpha,25-dihydroxyvitamin D3 was found 4 times less potent in binding to the porcine intestinal vitamin D receptor (VDR) and 2 times less effective in differentiation of HL-60 cells. 22-Iodinated vitamin D analogues showed somewhat lower in vitro activity, whereas (22,25)-epoxy analogues were inactive. Interestingly, it was established that (22S)-22-iodo-1alpha,25-dihydroxyvitamin D3 was 3 times more potent than its (22R)-isomer in binding to VDR and four times more effective in HL-60 cell differentiation assay. The restricted mobility of the side chain of both 22-iodinated vitamin D compounds was analyzed by a systematic conformational search indicating different spatial regions occupied by their 25-oxygen atoms. Preliminary data on the in vivo calcemic activity of the synthesized vitamin D analogues indicate that (E)-20(22)-dehydro-1alpha,25-dihydroxyvitamin D3 and 22-iodo-1alpha,25-dihydroxyvitamin D3 isomers were ca. ten times less potent than the natural hormone 1alpha,25-(OH)2D3 both in intestinal calcium transport and bone calcium mobilization.

Vitamin A antagonizes the action of vitamin D in rats

Interactions between vitamin A and vitamin D have been suggested for several decades but have not been established. In particular, vitamin A has been proposed to intensify the severity of the bone mineralization disease, rickets and inhibit the ability of vitamin D to cure this disease. To investigate this hypothesis, weanling Holtzman rats were fed a 1.2% calcium, 0.1% phosphorus diet and 15.5 ng ergocalciferol (vitamin D(2)) every 3 d for 21 d in the presence of increasing amounts of retinyl acetate (0 microg to 8621 microg/d). The increasing amounts of retinyl acetate produced a progressive and significant decrease in total bone ash (P < 0.001) and an increase in epiphyseal plate width (P < 0.001). The same experiment conducted with increasing amounts of vitamin D(2) (0 to 645 ng/d) indicated that the antagonism by retinyl acetate could be demonstrated at all vitamin D(2) dosages. To further investigate this antagonistic relationship, weanling Holtzman rats were fed a 0. 47% calcium, 0.3% phosphorus diet and 15.5 ng vitamin D(2) every 3 d for 33 d in the presence of increasing retinyl acetate (0 to 3448 microg/d). In the absence of retinyl acetate, these rats maintained a normal serum calcium level (2.34 mmol/L). Increasing retinyl acetate, however, eliminated the ability of vitamin D(2) to elevate the level of serum calcium (1.35 mmol/L). These results illustrated in vivo antagonism of vitamin D(2) action on intestine and bone by retinyl acetate.

Dietary calcium is a major factor in 1,25-dihydroxycholecalciferol suppression of experimental autoimmune encephalomyelitis in mice

The active form of vitamin D (1,25-dihydroxycholecalciferol) is a potent immune system regulator. Treating mice with 1, 25-dihydroxycholecalciferol and feeding them diets high in calcium can completely suppress the induction of experimental autoimmune diseases such as experimental autoimmune encephalomyelitis (EAE). Experiments described here were carried out on mice in which development of EAE was induced. Mice were fed diets containing various amounts of calcium and 1,25-dihydroxychole-calciferol. Variables measured were as follows: 1) incidence and severity of EAE; 2) serum calcium concentrations; 3) body weight; 4) total number of cells in the lymph nodes; and 5) interleukin-4 (IL-4) and transforming growth factor-beta1 (TGF-beta1) mRNA levels. When calcium was removed from the diet, the incidence of EAE was reduced 20% in both males and females. Further, the lower the dietary level of calcium, the higher was the dose of 1,25-dihydroxycholecalciferol required to prevent the symptoms. Thus, 1, 25-dihydroxycholecalciferol was found most effective in mice fed a diet adequate or high in calcium. 1,25-Dihydroxycholecalciferol treatment of mice fed high dietary calcium resulted in a decreased number of lymphocytes in the lymph nodes and increased IL-4 and TGF-beta1 mRNA levels. When calcium was omitted from the diet, 1, 25-dihydroxycholecalciferol supplementation increased TGF-beta1 mRNA. Increased IL-4 mRNA and decreased lymphocytes in the lymph nodes in response to 1,25-dihydroxycholecalciferol occurred only when dietary calcium was adequate or high. Our results suggest that dietary calcium and 1,25-dihydroxycholecalciferol are both involved in the prevention of symptomatic EAE.

A characterization of vitamin D-independent intestinal calcium absorption in the osteopetrotic (op/op) mouse

Previous work in our laboratory showed that the osteopetrotic (op/op) mouse possesses a vitamin D-independent mechanism of intestinal calcium absorption. This study was performed in an effort to further characterize the mechanism. The vitamin D-deficient op/op mouse absorbed calcium faster than either a vitamin D-deficient or 1, 25-dihydroxyvitamin D(3)-supplemented wild-type mouse. This increased rate of absorption was not found at concentrations of calcium that result in diffusional calcium absorption. Thus, vitamin D-deficient op/op mice had intestinal calcium absorption similar to that of vitamin D-deficient wild-type littermates when increasing levels of calcium were administered. Also, mRNA and protein levels of calbindin-D9k were similar in vitamin D-deficient wild-type and op/op mice as well as in wild-type and op/op mice treated with 1, 25-dihydroxyvitamin D(3). Therefore, the mechanism of vitamin D-independent intestinal calcium absorption in the op/op mouse is distinct from vitamin D-dependent intestinal calcium absorption.

DNA bending is induced by binding of vitamin D receptor-retinoid X receptor heterodimers to vitamin D response elements

The ability of vitamin D receptor-retinoid X receptor (VDR-RXR) heterodimers to induce a DNA bend upon binding to various vitamin D response elements (VDRE) has been investigated by circular permutation and phasing analysis. Recombinant rat VDR expressed in the baculovirus system and purified recombinant human RXR beta have been used. The VDREs were from 1,25-dihydroxyvitamin D3 (1,25-[OH]2D3) enhanced genes (rat osteocalcin, rOC; mouse osteopontin, mOP, and rat 1,25-dihydroxyvitamin D3-24-hydroxylase, r24-OHase), and a 1,25-(OH)2D3 repressed gene (human parathyroid hormone, hPTH). As shown by circular permutation analysis, VDR-RXR induced a distortion in DNA fragments containing various VDREs. Calculated distortion angles were similar in magnitude (57 degrees, 56 degrees, 61 degrees, and 59 degrees, respectively for rOC, mOP, r24-Ohase, and hPTH). The distortions took place with or without a 1,25-(OH)2D3 ligand. The centers of the apparent bend were found in the vicinity of the midpoint of all VDREs, except for rOC VDRE which was found 4 bp upstream. Phasing analysis was performed with DNA fragments containing mOP VDRE and revealed that VDR-RXR heterodimers induced a directed bend of 26 degrees, not influenced by the presence of hormone. In this study we report that similar to other members of the steroid and thyroid nuclear receptor superfamily, VDR-RXR heterodimers induce DNA bending.

Calcitonin is a major regulator for the expression of renal 25-hydroxyvitamin D3-1alpha-hydroxylase gene in normocalcemic rats

Regulation of vitamin D metabolism has long been examined by using vitamin D-deficient hypocalcemic animals. We previously reported that, in a rat model of chronic hyperparathyroidism, expression of 25-hydroxyvitamin D3-1alpha-hydroxylase (CYP27B1) mRNA was markedly increased in renal proximal convoluted tubules. It is believed that the major regulator for the expression of renal CYP27B1 is parathyroid hormone (PTH). However, in the normocalcemic state, the mechanism to regulate the renal CYP27B1 gene could be different, since plasma levels of PTH are very low. In the present study, the effect of PTH and calcitonin (CT) on the expression of renal CYP27B1 mRNA was investigated in normocalcemic sham-operated rats and normocalcemic thyroparathyroidectomized (TPTX) rats generated by either PTH or CaCl2 infusion. A single injection of CT dose-dependently decreased the expression of vitamin D receptor mRNA in the kidney of normocalcemic sham-TPTX rats. Concomitantly, CT greatly increased the expression of CYP27B1 mRNA in the kidney of normocalcemic sham-TPTX rats. CT also increased the expression of CYP27B1 mRNA in the kidney of normocalcemic TPTX rats. Conversion of serum [3H]1alpha,25(OH)2D3 from 25-hydroxy[3H]vitamin D3 in vivo was also greatly increased by the injection of CT into sham-TPTX rats and normocalcemic TPTX rats, but not into hypocalcemic TPTX rats. In contrast, administration of PTH did not induce the expression of CYP27B1 mRNA in the kidney of vitamin D-replete sham-TPTX rats and hypocalcemic TPTX rats. PTH increased the expression of renal CYP27B1 mRNA only in vitamin D-deficient hypocalcemic TPTX rats. These results suggest that CT plays an important role in the maintenance of serum 1alpha,25(OH)2D3 under normocalcemic physiological conditions, at least in rats.

Identification of a transcription factor that binds to the promoter region of the human parathyroid hormone gene

A putative transcription factor binds a site adjacent to the negative vitamin D responsive element (VDRE) in the promoter region of the human parathyroid hormone gene. Deletion and mutation analysis reveal the binding site for this factor overlaps with the proximal repeat element of the VDRE. It includes additional nucleotides at the 3' end of the VDRE. This site has the sequence TTTGAACCTATAGTTGAGAT and a core sequence TGAACCTAT needed for binding of the factor. Experiments with specific anti-vitamin D receptor (VDR) antibodies demonstrate that VDR is not found in the factor/DNA complex. However, removing the VDR from the nuclear extract by immunoprecipitation eliminated the binding complex, and the addition of recombinant VDR to the depleted extract did not restore the factor's ability to bind to the DNA, suggesting that the factor and VDR are closely associated. Transfection experiments with various reporter constructs indicate that the factor is required for the high transcriptional activity of the human PTH gene. This high activity is significantly suppressed by 1,25-dihydroxyvitamin D3. This factor seems to be expressed in several cell types including rat osteoblasts and pituitary. Additionally, some human cancer cell lines express a high level of this factor.

Three-dimensional structure-function relationship of vitamin D: side chain location and various activities

The various biological activities of side-chain mobility restricted analogs, four diastereomers at C(20) and C(22) of 22-methyl-1alpha,25-dihydroxyvitamin D3, were evaluated. The relationship between structure and the various activities of the analogs was discussed in terms of the active space region concept that we previously suggested.

Bacterial expression and characterization of the ligand-binding domain of the vitamin D receptor

The ligand-binding domain of the rat vitamin D receptor (amino acids 115-423) was expressed as an amino-terminal His-tagged protein in a bacterial expression system and purified over Ni-nitrilotriacetic acid resin and a Mono S column. The purified protein bound its ligand, 1,25-dihydroxyvitamin D3, with high affinity, similar to that of the full-length protein. Saturation of the protein with ligand quenched 90% of the tryptophan fluorescence, consistent with the purified protein being uniformly able to bind ligand. Addition of ligand produced no change in the tryptophan fluorescence lifetime, suggesting static quenching as the mechanism of fluorescence decrease. The near-UV circular dichroism spectrum showed a large increase in signal following the addition of ligand, consistent with a change in the environment of aromatic amino acid side chains. The far-UV circular dichroism spectrum was consistent with a protein of high alpha-helical content. Sedimentation equilibrium experiments demonstrated that the protein formed higher-order complexes, and the distribution of the protein among these complexes was significantly shifted by addition of ligand.

Additional protein factors play a role in the formation of VDR/RXR complexes on vitamin D response elements

The vitamin D receptor (VDR) elicits a transcriptional response to 1,25-dihydroxyvitamin D3 by binding to specific response elements (VDRE) in the promoter of target genes. Retinoic X receptor (RXR) is required for formation of the VDR-VDRE complex when VDR is supplied at physiologic concentrations. When porcine intestinal nuclear extract is used as a source of VDR, two distinct complexes are always observed with native gel electrophoresis. Both complexes contain VDR and RXR. We now show that the faster-migrating complex requires another heretofore unknown nuclear factor for its formation. In addition, we provide evidence that the formation of the slower-migrating complex is enhanced by transcription factor IIB (TFIIB). Using ligand binding assays, we determined that both complexes contain the same ratio of VDR to VDRE. Using RXR subtype-specific antibodies in gel shift assays, we show that the complexes contain more than one RXR subtype. Therefore, the present results demonstrate VDR-RXR-VDRE complexes formed with pig intestinal nuclear extracts contain other proteins and that the complexes formed between VDR and VDRE are not simply heterodimers of VDR and RXR.

Defects in embryonic hindbrain development and fetal resorption resulting from vitamin A deficiency in the rat are prevented by feeding pharmacological levels of all-trans-retinoic acid

Vitamin A is required for reproduction and normal embryonic development. We have determined that all-trans-retinoic acid (atRA) can support development of the mammalian embryo to parturition in vitamin A-deficient (VAD) rats. At embryonic day (E) 0.5, VAD dams were fed purified diets containing either 12 micrograms of atRA per g of diet (230 micrograms per rat per day) or 250 micrograms of atRA per g of diet (4.5 mg per rat per day) or were fed the purified diet supplemented with a source of retinol (100 units of retinyl palmitate per day). An additional group was fed both 250 micrograms of atRA per g of diet in combination with retinyl palmitate. Embryonic survival to E12.5 was similar for all groups. However, embryonic development in the group fed 12 micrograms of atRA per g of diet was grossly abnormal. The most notable defects were in the region of the hindbrain, which included a loss of posterior cranial nerves (IX, X, XI, and XII) and postotic pharyngeal arches as well as the presence of ectopic otic vesicles and a swollen anterior cardinal vein. All embryonic abnormalities at E12.5 were prevented by feeding pharmacological amounts of atRA (250 micrograms/g diet) or by supplementation with retinyl palmitate. Embryos from VAD dams receiving 12 micrograms of atRA per g of diet were resorbed by E18.5, whereas those in the group fed 250 micrograms of atRA per g of diet survived to parturition but died shortly thereafter. Equivalent results were obtained by using commercial grade atRA or atRA that had been purified to eliminate any potential contamination by neutral retinoids, such as retinol. Thus, 250 micrograms of atRA per g of diet fed to VAD dams (approximately 4.5 mg per rat per day) can prevent the death of embryos at midgestation and prevents the early embryonic abnormalities that arise when VAD dams are fed insufficient amounts of atRA.

New 1alpha,25-dihydroxy-19-norvitamin D3 compounds of high biological activity: synthesis and biological evaluation of 2-hydroxymethyl, 2-methyl, and 2-methylene analogues

New highly active isomers of the natural hormone 1alpha, 25-dihydroxyvitamin D3 possessing an exomethylene group at the 2-position were prepared in a convergent manner, starting with (-)-quinic acid and the corresponding (20R)- and (20S)-25-hydroxy Grundmann ketones. These 2-methylene-19-norvitamins were efficiently converted to the 2-methyl and 2-hydroxymethyl derivatives, some of which exhibited pronounced in vivo biological activity. Configurations of the A-ring substituents were determined by 1H NOE difference spectroscopy as well as by spin decoupling experiments. It was established that the bulky methyl and hydroxymethyl substituents at C-2, due to their large conformational free energies, occupy mainly equatorial positions. Additionally, hydroxylation of the C(10)-C(19) double bond in 1alpha,25-(OH)2D3 was performed, resulting in 1alpha,19,25-trihydroxy-10,19-dihydrovitamin D3 derivatives in which the hydroxymethyl substituent at C-10, for steric reasons, is forced to occupy an axial position. In consequence, the vitamin D3 analogues were synthesized in which the 1alpha-hydroxy group, required for biological activity, is almost exclusively axially or equatorially oriented because of stabilization of the single A-ring chair conformations. The relative ability of the synthesized analogues to bind the porcine intestinal vitamin D receptor was assessed and compared with that of the natural hormone. It was established that vitamins possessing the axial orientation of the 1alpha-hydroxy substituent exhibit a significantly increased receptor binding affinity. Compounds with a 2-methylene substituent showed selective calcemic activity profiles, being extremely effective on bone calcium mobilization. 2alpha-Methyl-substituted vitamins proved to be much more active in vivo than the corresponding epimers with 2beta-configuration. All of the 2-substituted vitamins exhibited pronounced HL-60 differentiating activity, those 2alpha-substituted in the 20S-series being especially potent. The present studies imply that the axial orientation of the 1alpha-hydroxy group is necessary for biological activity of vitamin D compounds.

Current understanding of the molecular actions of vitamin D

The important reactions that occur to the vitamin D molecule and the important reactions involved in the expression of the final active form of vitamin D are reviewed in a critical manner. After an overview of the metabolism of vitamin D to its active form and to its metabolic degradation products, the molecular understanding of the 1alpha-hydroxylation reaction and the 24-hydroxylation reaction of the vitamin D hormone is presented. Furthermore, the role of vitamin D in maintenance of serum calcium is reviewed at the physiological level and at the molecular level whenever possible. Of particular importance is the regulation of the parathyroid gland by the vitamin D hormone. A third section describes the known molecular events involved in the action of 1alpha,25-dihydroxyvitamin D3 on its target cells. This includes reviewing what is now known concerning the overall mechanism of transcriptional regulation by vitamin D. It describes the vitamin D receptors that have been cloned and identified and describes the coactivators and retinoid X receptors required for the function of vitamin D in its genomic actions. The presence of receptor in previously uncharted target organs of vitamin D action has led to a study of the possible function of vitamin D in these organs. A good example of a new function described for 1alpha,25-dihydroxyvitamin D3 is that found in the parathyroid gland. This is also true for the role of vitamin D hormone in skin, the immune system, a possible role in the pancreas, i.e., in the islet cells, and a possible role in female reproduction. This review also raises the intriguing question of whether vitamin D plays an important role in embryonic development, since vitamin D deficiency does not prohibit development, nor does vitamin D receptor knockout. The final section reviews some interesting analogs of the vitamin D hormone and their possible uses. The review ends with possible ideas with regard to future directions of vitamin D drug design.

Combinational effects of vitamin D3 and retinoic acid (all trans and 9 cis) on proliferation, differentiation, and programmed cell death in two small cell lung carcinoma cell lines

The effects of a combination of vitamin D3 [1,25(OH)2D3] and retinoic acid (RA) on proliferation, differentiation, and apoptosis of the human small cell lung carcinoma (SCLC) cell lines NCI-H82 and NCI-H209 were evaluated. Cell proliferation was inhibited by 1,25(OH)2D3 and RA alone. The combination of 1,25(OH)2D3 and the cis form of retinoic acid resulted in an additive decrease in cell proliferation and the induction of apoptosis in various concentrations. Moreover, 3H-thymidine incorporation was inhibited and the number of viable cells was decreased. The characteristics of the apoptotic cells were examined and confirmed by morphologic analysis, light and electron microscopy, and fluorescence detection. It was concluded that 1,25(OH)2D3 and RA exert additive effects on the inhibition of proliferation and the induction of apoptosis in both the NCI-H82 and the NCI-H209 SCLC cell lines. This finding has important implications for the use of retinoids and 1,25(OH)2D3 in cancer prevention and in the therapy of small cell lung carcinoma.

Inhibition of 25-hydroxyvitamin D3 production by 1, 25-dihydroxyvitamin D3 in rats

The regulation of the hepatic vitamin D3 25-hydroxylase enzyme system by 1,25-dihydroxyvitamin D3 was examined using male rats. Circulating 25-hydroxyvitamin D3 levels decreased in response to increasing doses of 1,25-dihydroxyvitamin D3 as determined by HPLC and serum protein binding assay. A maximum reduction in serum 25-hydroxyvitamin D3 of 57.7% was achieved by the administration of 1,25-dihydroxyvitamin D3 and higher doses produced no further reduction. The in vitro rate of vitamin D 25-hydroxylation by liver homogenates from rats given 1,25-dihydroxyvitamin D3 was also reduced by 60.4%. Whole rat extracts from animals given [3H]vitamin D3 contained 50.4% less 25-[3H]hydroxyvitamin D3 in treated rats given 1,25-dihydroxyvitamin D3 than in those given only the vehicle. Further, 1,25-dihydroxyvitamin D3 treatment had no effect on in vivo disappearance of 25-[3H]hydroxyvitamin D3. Taken together, these results demonstrate that the decrease in 25-hydroxyvitamin D3 levels observed in 1,25-dihydroxyvitamin D3-treated rats results from a decrease in production and not an increase in the metabolic clearance of 25-[3H]hydroxyvitamin D3.

1,25-dihydroxyvitamin D3 is a positive regulator for the two anti-encephalitogenic cytokines TGF-beta 1 and IL-4

Previously we demonstrated that 1,25-dihydroxyvitamin D3 blocks the progression of relapsing encephalomyelitis. We now propose that 1,25-dihydroxyvitamin D3 blocks these autoimmune symptoms by stimulating the differentiation and/or function of cells that inhibit the encephalitogenic process. To support this belief, we have found that 1,25-dihydroxyvitamin D3 administration to mice increases IL-4 transcripts by 3- to 25-fold and TGF-beta 1 transcripts by 4- to 24-fold. Similarly, IL-4 and TGF-beta 1 transcripts were higher in the central nervous system of 1,25-dihydroxyvitamin D3-treated mice compared with controls. The number of cells recoverable from the lymph nodes of 1,25-dihydroxyvitamin D3-treated mice was only 50% that of controls. Overall, 1,25-dihydroxyvitamin D3 treatment causes a net loss in the total number of lymphocytes while the number of IL-4 and TGF-beta 1 transcripts increased. The systemic and local increase in the expression of these two anti-inflammatory cytokines by 1,25-dihydroxyvitamin D3 may be responsible for the ability of this drug to block encephalomyelitis.

Identification of a new all-trans-retinol metabolite produced through a new retinol metabolic pathway

In vitro incubation of all-trans-retinol (atROL) with kidney homogenate from vitamin A-deficient and retinoic acid-supplemented (VAD-RAS) female rats produces a new retinol metabolite. Reverse-phase (RP) and normal-phase (NP) high-performance liquid chromatography (HPLC) analysis showed that this metabolite coelutes with the unknown all-trans-retinol (atROL) metabolite previously found in the day 10 conceptus and kidneys of vitamin A-deficient rats maintained on all-trans-retinoic acid (VAD-RA) and given 2 microg of [3H]atROL. Normal-phase (NP) HPLC purification of the metabolite collected from a RP HPLC column further separated the radiolabeled material into two components. The two isolated compounds have identical or very similar spectroscopic properties. Their nuclear magnetic resonance (1H NMR) and mass spectra (MS) indicated that they are isomers. Spectroscopic studies of the metabolites and their derivatives showed that they are nine-carbon fragments resulting from an oxidative cleavage of the side chain of atROL. The cleavage occurs at C-9, and the product is then oxidized to a keto group. The primary hydroxy group from atROL is preserved in the metabolite. A sulfide bridge is formed between C-11 and C-14, which interrupts the conjugation. The formation of the new metabolites, possessing a 2,5-dihydrothiophene ring, is catalyzed by an enzyme(s) located in the cytosolic fraction of kidneys. The process represents a new retinol metabolic pathway; however, its biological significance is unknown.

Parathyroid hormone activation of the 25-hydroxyvitamin D3-1alpha-hydroxylase gene promoter

The DNA flanking the 5' sequence of the mouse 1alpha-hydroxylase gene has been cloned and sequenced. A TATA box has been located at -30 bp and aCCAAT box has been located at -79 bp. The gene's promoter activity has been demonstrated by using a luciferase reporter gene construct transfected into a modified pig kidney cell line, AOK-B50. Parathyroid hormone stimulates this promoter-directed synthesis of luciferase by 17-fold, whereas forskolin stimulates it by 3-fold. The action of parathyroid hormone is concentration-dependent. 1,25-Dihydroxyvitamin D3 does not suppress basal promoter activity and marginally suppresses parathyroid hormone-driven luciferase reporter activity. The promoter has three potential cAMP-responsive element sites, and two perfect and one imperfect AP-1 sites, while no DR-3 was detected. These results indicate that parathyroid hormone stimulates 25-hydroxyvitamin D3-1alpha-hydroxylase by acting on the promoter of the 1alpha-hydroxylase gene.